1. Field of the Invention
This invention relates to formation of high dielectric constant capacitor structures comprising amorphously frustrated ferroelectric materials deposited by metalorganic chemical vapor deposition (MOCVD).
2. Description of the Related Art
In recent years, ferroelectric materials such as Pb(Zr,Ti)O3 and SrBi2Ta2O9 have been the focus of widespread interest as components of non-volatile memory devices. Non-volatile ferroelectric memory devices effect information storage by polarization of a thin ferroelectric material layer disposed between two plates of a capacitor structure. Each such ferroelectric capacitor is connected to a transistor to form a storage cell, in which the transistor controls the access of the read-out electronics to the capacitor. The transistor therefore is connected to bit-line and word-line elements, to constitute the storage cell.
The ferroelectric material may be utilized in a stacked capacitor structure that overlies the top of the transistor. The transistor drain (e.g., of a MOSFET structure) is connected to the bottom electrode of the capacitor by a plug formed of a suitable material such as polysilicon or tungsten.
Information subsequently can be changed in the ferroelectric memory cell by applying an electric field to the thin ferroelectric material layer to reverse (xe2x80x9cflipxe2x80x9d) the polarization characteristic of the ferroelectric material. Ferroelectric memories (FRAMs), in contrast to dynamic random access memories (DRAMs), have the advantages of retaining stored information in the event of termination of the power supply, and do not require refresh cycles.
In such memory applications, ferroelectric materials desirably have the following electrical properties: (1) a low coercive field characteristic, facilitating use of a low voltage power supply; (2) a high remanent polarization characteristic, ensuring highly reliable information storage; (3) absence of significant fatigue or life-time deterioration characteristics; (4) absence of any imprint which would alter the stored information (e.g., leading to a preference of a certain polarization such as a logical xe2x80x9c1xe2x80x9d over a logical xe2x80x9c0xe2x80x9d character) or otherwise impair the ability to xe2x80x9creadxe2x80x9d the stored information; and (5) extended retention time, for reliable data storage over an extended period of time.
The foregoing electrical property criteria are satisfied in the layered pseudo-perovskite or xe2x80x9cAurivilliusxe2x80x9d phase of materials such as strontium bismuth tantalate, SrBi2Ta2O9, sometimes hereinafter referred to as xe2x80x9cSBT.xe2x80x9d As a result of these favorable characteristics, significant efforts have been initiated to integrate SBT in memory devices.
Ferroelectric capacitor structures utilizing SBT as the ferroelectric material have been made in the prior art by sol-gel techniques and demonstrate superior electrical properties. Nonetheless, the sol-gel methodology of forming ferroelectric thin films for this application permits only a low integration density to be achieved. Some improvement in the sol-gel methodology may be gained by mist or electrospray methods, permitting fabrication of memories up to 4 megabit in capacity. To achieve higher integration density of ferroelectric thin films of materials such as Pb(Zr,Ti)O3 (PZT) and SrBi2Ta2O9 (SBT) with smaller structure sizes (e.g., having a minimal feature size below about 0.7 micron), it is necessary to utilize chemical vapor deposition (CVD) processes, since CVD affords better conformality and step coverage than layers produced by any other deposition method. Further, the CVD process yields deposited films having high film uniformity and high film density, with the capability to grow very thin films at high throughput and low cost.
The art continues to seek improvement in high dielectric constant materials for thin film capacitor structures and ferroelectric memory applications, and in improved processes for the formation of such materials, as for example CVD processes for the deposition of bismuth oxide materials (e.g., SrBi2Ta2O9, Bi4Ti3O12, etc).
Among dielectric materials, thin film xe2x80x9cfrustrated ferroelectricsxe2x80x9d or xe2x80x9csuperparaelectricsxe2x80x9d such as (Ba,Sr)TiO3 (BST) have extremely high dielectric constants. Such materials are ferroelectric in bulk form, but are paraelectric in thin film form. The mechanism of xe2x80x9cfrustratingxe2x80x9d or suppressing the ferroelectric switching in these films is a field of current research, however, the resulting capacitor structures have high capacitance, large changes in capacitance with bias voltage, and low loss. These materials continue to be developed for high-density DRAM devices and high frequency tunable devices.
The aforementioned paraelectric thin film materials are deposited by MOCVD at temperatures above 500xc2x0 C., which however is too high for many backend processes used in semiconductor processing. These paraelectric thin films also exhibit large changes in dielectric constant with bias voltage, a property quite useful for making tunable devices, but not for other applications where stable capacitance is desired. The dielectric constant also changes significantly with changes in temperature.
Taken against the background of the above-discussed state of the art, there is a continued need for improved low temperature, high dielectric constant capacitors.
The present invention relates in one aspect to an amorphous metal oxide thin film material of a composition that is ferroelectric in both crystalline thin film and crystalline bulk forms. The material comprises the elements of the corresponding ferroelectric composition, and has a same stoichiometry as the ferroelectric material.
The invention also contemplates amorphous metal oxide thin film materials compositionally related to such ferroelectric compositions, but which are xe2x80x9caway fromxe2x80x9d the ferroelectric material, i.e., off-stoichiometric in relation thereto. Such xe2x80x9caway fromxe2x80x9d ferroelectric compositions may for example, in the case of SBT, have metal site ratios of from about 0 to 20% Sr, 5 to 70% Bi and 10 to 95% tantalum or 0 to 10% strontium, 5 to 55% bismuth and 35 to 95% tantalum or 0 to 5% strontium, 15 to 50% bismuth and 45 to 85% tantalum.
The optimum composition for such amorphous dielectric film contains the elements of the bulk ferroelectric with a stoichiometry that is significantly away from the optimum ferroelectric stoichiometry. This same behavior has been observed in the case of Baxe2x80x94Srxe2x80x94Tixe2x80x94O as described in U.S. Pat. No. 5,932,905, wherein compositions from 65-90% Ti on the metal site produced the best combination of capacitance, leakage, and linearity. Such phenomena is general and for each ferroelectric composition herein aforementioned, a highly favorable amorphous dielectric composition is located nearby.
Thin film amorphous materials of the invention have a voltage independent capacitance, a capacitance density in the range of from about 1000 to about 10000 nF/cm2, and a current leakage of  less than 10xe2x88x927 A/cm2.
Another aspect of the invention relates to a thin film capacitor microelectronic device structure, comprising:
a first electrode;
a dielectric material selected from the group consisting of amorphous metal oxide thin film materials of a composition that is ferroelectric in both crystalline thin film and crystalline bulk forms, and amorphous metal oxide thin film materials related to ferroelectric compositions but deviating from the stoichiometry of the ferroelectric material, wherein the dielectric material in thin film amorphous form has a voltage independent capacitance, a capacitance density in the range of from about 1000 to about 10000 nF/cm2, and a current leakage of  less than 10xe2x88x927 A/cm2; and
a second electrode.
A still further aspect of the invention relates to a method of forming a thin film capacitor structure on a substrate, comprising:
forming a first electrode on the substrate;
depositing on the first electrode a dielectric material selected from the group consisting of amorphous metal oxide thin film materials of a composition that is ferroelectric in both crystalline thin film and crystalline bulk forms, and amorphous metal oxide thin film materials related to ferroelectric compositions but deviating from the stoichiometry of the ferroelectric material, wherein the dielectric material in thin film amorphous form has a voltage independent capacitance, a capacitance density in the range of from about 1000 to about 10000 nF/cm2, and a current leakage of  less than 10xe2x88x927 A/cm2, and
forming a second electrode on the layer of amorphous metal oxide thin film material.
Other objects, features and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.
As used herein, the term xe2x80x9cthin filmxe2x80x9d means a material layer thickness below 10,000 nanometers.
The term xe2x80x9chigh kxe2x80x9d as used herein means a dielectric constant above 30.